Tóth-Petróczy Agnes, Oldfield Christopher J, Simon István, Takagi Yuichiro, Dunker A Keith, Uversky Vladimir N, Fuxreiter Monika
Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, Budapest, Hungary.
PLoS Comput Biol. 2008 Dec;4(12):e1000243. doi: 10.1371/journal.pcbi.1000243. Epub 2008 Dec 19.
The Mediator complex provides an interface between gene-specific regulatory proteins and the general transcription machinery including RNA polymerase II (RNAP II). The complex has a modular architecture (Head, Middle, and Tail) and cryoelectron microscopy analysis suggested that it undergoes dramatic conformational changes upon interactions with activators and RNAP II. These rearrangements have been proposed to play a role in the assembly of the preinitiation complex and also to contribute to the regulatory mechanism of Mediator. In analogy to many regulatory and transcriptional proteins, we reasoned that Mediator might also utilize intrinsically disordered regions (IDRs) to facilitate structural transitions and transmit transcriptional signals. Indeed, a high prevalence of IDRs was found in various subunits of Mediator from both Saccharomyces cerevisiae and Homo sapiens, especially in the Tail and the Middle modules. The level of disorder increases from yeast to man, although in both organisms it significantly exceeds that of multiprotein complexes of a similar size. IDRs can contribute to Mediator's function in three different ways: they can individually serve as target sites for multiple partners having distinctive structures; they can act as malleable linkers connecting globular domains that impart modular functionality on the complex; and they can also facilitate assembly and disassembly of complexes in response to regulatory signals. Short segments of IDRs, termed molecular recognition features (MoRFs) distinguished by a high protein-protein interaction propensity, were identified in 16 and 19 subunits of the yeast and human Mediator, respectively. In Saccharomyces cerevisiae, the functional roles of 11 MoRFs have been experimentally verified, and those in the Med8/Med18/Med20 and Med7/Med21 complexes were structurally confirmed. Although the Saccharomyces cerevisiae and Homo sapiens Mediator sequences are only weakly conserved, the arrangements of the disordered regions and their embedded interaction sites are quite similar in the two organisms. All of these data suggest an integral role for intrinsic disorder in Mediator's function.
中介体复合物在基因特异性调控蛋白与包括RNA聚合酶II（RNAP II）在内的通用转录机制之间提供了一个接口。该复合物具有模块化结构（头部、中部和尾部），冷冻电子显微镜分析表明，它在与激活剂和RNAP II相互作用时会发生显著的构象变化。这些重排被认为在起始前复合物的组装中起作用，也有助于中介体的调控机制。与许多调控和转录蛋白类似，我们推测中介体也可能利用内在无序区域（IDR）来促进结构转变并传递转录信号。事实上，在酿酒酵母和智人的中介体各个亚基中都发现了高比例的IDR，尤其是在尾部和中部模块。从酵母到人类，无序程度增加，尽管在这两种生物体中，它都显著超过了类似大小的多蛋白复合物。IDR可以通过三种不同方式促进中介体的功能：它们可以单独作为具有独特结构的多个伙伴的靶位点；它们可以作为连接球状结构域的可塑接头，赋予复合物模块化功能；它们还可以响应调控信号促进复合物的组装和解聚。在酵母和人类中介体的16个和19个亚基中分别鉴定出了短片段的IDR，称为分子识别特征（MoRF），其特点是具有高蛋白-蛋白相互作用倾向。在酿酒酵母中，11个MoRF的功能作用已通过实验验证，Med8/Med18/Med20和Med7/Med21复合物中的MoRF在结构上得到了证实。尽管酿酒酵母和智人的中介体序列仅具有微弱的保守性，但在这两种生物体中，无序区域及其嵌入的相互作用位点的排列非常相似。所有这些数据表明内在无序在中介体功能中起着不可或缺的作用。
